Evolutionary physiologists have long been interested in physiological mechanisms underpinning individual variation in energy metabolism and performance. Recent efforts to elucidate these mechanisms have largely focused on bioenergetics and oxidative stress. One underappreciated area that could play a role in mediating individual variation in performance is the unfolded protein response (UPR); a cellular stress response that during its adaptive phase reduces secretory protein load, enhances endoplasmic reticulum (ER) protein folding, and increases clearance capacity during homeostasis dysregulation and stress. Given that the ER and mitochondria interact closely to regulate cellular homeostasis, it seems intuitive that UPR phenotype would correlate strongly with mitochondrial physiology, which in turn would contribute to variations in whole-organism metabolism. This study aims to explore the relationship between UPR phenotype, cellular respiration, and whole-organism metabolism in deer mice (Peromyscus maniculatus). 33 mice were phenotyped at weaning for UPR using fibroblasts cultured from ear punches. Animals were then categorized into high UPR responders and low UPR responders. We acclimated animals to a semi-natural environment for 6 months, during which resting metabolic rate was measured. Upon termination of the study, animals were sacrificed to obtain lung tissues for fibroblast culture. Cellular respiration measurements were conducted on lung fibroblasts. Preliminary results showed that resting metabolic rates were similar between high- and low- UPR responders. Cellular metabolic rates and were marginally higher in high UPR responders. Data on cellular stress resistance will be presented.